1. Field of the Invention
The present invention relates to apparatus for raising and lowering heavy equipment, and more particularly, the invention relates to a hoist rig for launching and recovering a pipestring in the operation of a deep ocean mining vessel.
2. Description of the Prior Art
The potential of the ocean for supplying important and basic raw materials is generally recognized. Mining operations for sand, gravel, shell and other materials from continental shelf deposits are presently being performed by dredging techniques. On the ocean floor in deeper waters are vast quantities of mineral deposits. Among these deposits are mineral concentrations spread over large areas of the ocean floor in the form of nodules. Existence of nodules on the ocean bottom has been known for many years and are believed to be formed over aeons of time due to the precipitation of the mineral substances out of the seawater. These nodules are known to consist essentially of iron oxide, manganese oxide, copper, cobalt and nickel, and are generally found in the deep areas of the sea where the floor is relatively hard and flat. The areas in which the nodules are presently known to exist in sufficient quantities to sustain a profitable mining operation are found generally more than 200 miles off shore and at depths of up to 18,000 feet and more.
Among the numerous systems which have been conceived for the recovery of nudules from the ocean floor is the hydraulic system which generally consists of a pipestring which is suspended from a floating platform or vessel. The system includes a gathering head which is designed to collect and winnow the nodules from the ocean floor sediments and transport them through the pipestring. Means are provided for causing the water inside the pipestring to flow upward with sufficient velocity to draw the nodules into the system and transport them to the surface.
One of the major problems associated with this mining method is the provision of hoist means for launching and recovering the pipestring. The load of the pipestring for working in depths up to 18,000 feet may exceed 5,000 kips. Since the design load of the hoist apparatus must include a conservative safety factor, it will be appreciated that a hoist rig for handling the pipestring in such deep ocean mining operations must be capable of lifting unusually large loads. Such large loads exceed the load lifting capability of conventional hoist apparatus. Generally, the conventional hoist rigs include a single derrick or mast support and one or more cables reeved on sheave assemblies. The theoretical limit of the magnitude of the load which can be lifted by such an arrangement depends upon the structural strength of the mast structure and the tension rating of the cables which reeved around the support sheaves. The tension strength of the cables is generally proportional to the diameter of the cables. However, as the diameter of the cables is increased to lift heavier loads, the flexibility of the cables decreases. The cables, of course, must be strong enough to support the load of the pipestring and must also be flexible enough to permit the traveling block, which engages the pipestring, to be transported smoothly and rapidly for efficient operation during launching or recovery operations.
Recent improvements in the construction of mast structures has greatly increased the maximum load bearing capability of a single mast structure. For example, an improved mast structure is disclosed and claimed in U.S. Pat. No. 3,960,360 to Thomas L. Elliston. The dynamic load efficiency of that mast structure is substantially greater than conventional mast structures of comparable size and has performed entirely satisfactorily for lifting loads in the intermediate range. The maximum load bearing capability of such a structure can be designed to match the load bearing capability of multiple runs of power transmission cables which are reeved in sheaves supported by the mast structure. However, because of the flexibility constraint discussed above, the diameter of the power transmission cable cannot be increased substantially without compromising the efficiency and execution of launching and recovery operations. Furthermore, the number of power transmission cable runs is also limited because of the relatively small amount of cable running space available along a single mast structure. It is, therefore, a principal object of the present invention to provide an improved hoist rig for accommodating the substantially greater load handling requirements associated with the launching and recovering of pipestrings in deep ocean mining operations.
Examples of prior art approaches for improving the load handling ability of a hoist rig having sheaves and cables reeved in the sheaves are disclosed in the following U.S. Pat. Nos. 1,928,958 to a Young et al.; 2,239,493 to Nichols; 2,766,009 to Wilson; and 3,719,238 to Campbell et al.